Silicon finds wide application in electronic device production. One particular utility is thin film transistors (TFTs). Such transistors are critical components in numerous devices. A device of great interest currently is a liquid crystal display (LCD).
It is a goal of the silicon device industry, and especially active matrix liquid crystal device (AMLCD) production, to achieve higher performance by replacing amorphous silicon (a-Si) thin film transistors (TFTs) with a polycrystalline silicon (poly-Si) version. This change would result in higher on-currents, better frequency response, larger displays, and potentially longer life-times. It would be possible to implement peripheral circuitry, as well as pixel switches, on the glass substrate. This would greatly reduce the number of external connections to the substrate, thus increasing reliability and reducing cost.
Poly-Si TFTs are very attractive for the above reasons. The poly-Si film required for TFT fabrication can be formed directly on a flat glass substrate with a high strain point, such as Corning Code 1737 glass. However, these poly-Si TFTs are typically fabricated on glass substrates having an intermediate SiO.sub.2 barrier layer deposited thereon between the base poly-Si layer and the glass substrate. This SiO.sub.2 barrier coating is usually between 0.5 and 1 .mu.m thick. The reason that a SiO.sub.2 barrier layer is used is two fold. First the layer serves to separate the glass components, such as Al and B, from the active poly-Si base layer. This separation is critical because Al and B act as dopants in Si and would create a conductive back surface, hence increasing the leakage current of the transistor. The second use of the barrier layer is to prevent alkali diffusion out of the glass and into the subsequent device electronics. Such barrier layers are discussed, for example, in "Sodium Redistribution Between Oxide Phases", Araujo and Fehiner, J. Non-Crystalline Solids, Vol. 197, pp. 154-163, and "Barrier Layers for Poly-Si TFT Displays", Moore et al., SID 96 DIGEST, pp. 543-545 (1996). These articles indicate that by placing an SiO.sub.2 barrier layer on an aluminoborosilicate glass the sodium redistribution will go against a concentration gradient. The sodium will tend to migrate from the low Na SiO.sub.2 barrier layer into a higher Na concentration glass substrate.
One challenge to using poly-Si TFTs for active matrix pixel transistors is that poly-Si TFTs usually have higher currents in the off-state (leakage current) than do a-Si TFTs. Therefore, to address the active matrix display without losing contrast, the pixel TFTs should preferably have leakage currents less than 1.times.10.sup.-10 amps. It would be desirable to eliminate the SiO.sub.2 deposition process, and be able to deposit the TFT film directly onto the glass surface. However, as indicated in the "barrier layers" article, such attempts have heretofore resulted in unacceptable leakage currents.